To figure this out, scientists at the University of California, Berkeley and Harvard studied temperature data from 1850 to 2007 compiled by the University of East Anglia's Climate Research Unit in the United Kingdom.

They found that temperatures over land in the 100-year period between 1850 and 1950 showed a simple, natural pattern of variability, with the hottest day of the year in the Northern Hemisphere landing around July 21. But from the mid-1950s onward (the period when global average temperatures began to rise), the hottest day came 1.7 days earlier.

This shift is happening at the same time that those summer and winter peaks are getting warmer and the gap between them is closing (because winter temperatures are rising faster than summer ones).

And with this shift of peak warming and cooling comes a corresponding shift in the onset of the seasons, which the researchers say explains the month-to-month pattern of temperatures over the past 50 years.

"Once we have accounted for the fact that the temperature averaged over any given year is increasing, we find that some months have been warming more than other months," said Alexander Stine, a graduate student at UC Berkeley. "We were surprised to find that over land, most of the difference in the warming of one month relative to another is simply the result of this shift in the timing of the seasons, and a decrease in the difference between summer and winter temperatures."

The timing of the shift along with the rise in global temperatures leads Stine and his colleagues to think that human-caused climate change is the ultimate cause behind the shift. But exactly which effects of global warming are driving the shift is less clear.

Stine and his colleagues think the shift in seasons is due in part to a particular pattern of winds, known as the Northern Annular Mode, which has also been changing over the same time period. A change in the direction and strength of the winds can move heat from the ocean onto land, which may affect the timing of the seasons. But the relationship between this wind pattern and seasonal shift isn't strong enough to explain the full magnitude of the shift.

Other possible influences the team is looking into are drier global soils, which would cause the land surface to heat more rapidly in response to the sun's rays, and changes in the amount of solar energy absorbed by the atmosphere as a result of industrial pollution.

The research, detailed in the Jan. 22 issue of the journal Nature, was funded by the National Science Foundation.